1. Field of the Invention
The present invention relates generally to ink-jet hard copy printing and, more particularly to devices and techniques for improving the print quality of multiple pen, or ink cartridge, color ink-jet hard copy apparatus.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988). For convenience of description, hard copy apparatus of all shapes, sizes, and varieties are referred to hereinafter simply as a "printer."
FIG. 1 (PRIOR ART) depicts an ink-jet, computer peripheral printer 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101 (with its housing shell removed). Operation is administrated by an electronic controller 102, usually a microprocessor or printed circuit board connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC's. Cut-sheet print media 105, loaded by the end-user onto an input tray 120, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or printing zone, 107 where graphical images or alphanumeric text is created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113 is provided for keeping track of the position of the carriage 109 at any given time. At least one, or a set, of individual ink-jet pens, or print cartridges, 115.sub.x (where x=an ink-type specification) are releasable mounted in the carriage 109 for easy access. Generally, in a full color system, inks for the subtractive primary colors--cyan, yellow, magenta (CYM) and true black (K) (F standing for a fixer fluid sometimes used to enhance ink drying time (see e.g., U.S. Pat. No. 5,635,969 by Allen for a METHOD AND APPARATUS FOR THE APPLICATION OF MULTIPART INK-JET INK CHEMISTRY, assigned to the common assignee of the present invention and incorporated herein by reference))--are provided in remote, or "off-axis," replaceable or refillable, ink reservoirs 117.sub.x having fluidic couplings 119 to the pens 115.sub.x. Once a printed page is completed, the print medium is ejected onto an output tray 121.
It is common in the art to refer to the pen scanning direction as the Cartesian x-axis, the paper feed direction as the y-axis, and the ink drop firing direction as the z-axis. For convenience of description, print media of all shapes, sizes, and varieties are referred to hereinafter simply as "paper." An image is formed by printing a pattern of individual dots at particular locations of an array defined for the sheet of paper positioned in the print zone 107. These locations are conveniently visualized as being in a rectilinear matrix and are referred to synonymously as picture elements, or "pixels," or "dot locations (or positions)" or "drop locations (or positions)." The printing operation is thus the filling in of pixels identified in a print data set with dots, or drops, of ink. Primary, or "base," colors are produced at a pixel by depositing one or more drops from a respective pen 115.sub.x and other hues are produced my depositing multiple drops of different base color inks onto the same pixel, the overprinting of two or more base colors producing secondary colors according to well-established optical principles.
Print quality is determined by dot-resolution and the precise placement of drops, particularly of over-printed drops which occur on separate scans of a swath of print to allow sufficient drying time of a first drop deposition prior to deposit of the overlying drop. As current commercial product resolution is about 300 dots per inch ("dpi"), and preferred resolution is more than 600 dpi, the problem of accurate drop placement places a constant demand for the designer also striving to meet the commercial demand for greater throughput (pages per minute, "ppm") and increased color rendition, particularly in photographic quality ink-jet printing.
One prior art technique to improve print quality by allowing longer dry time for first deposition ink drops is taught in U.S. Pat. No. 5,316,958, filed on May 1, 1992 by Richtsmeier, Doan and Hickman (common applicant herein) for STAGGERED PENS IN COLOR THERMAL INK-JET PRINTER (Richtsmeier et al.). A four pen system is shown which offsets different color printheads such that nozzle arrays traverse spaced apart non-overlapping print regions as the carriage is scanned along the carriage scan axis. This system allows longer first deposition drying time, but requires a full print cartridge width in the product. Simultaneous, multiple printhead use per color is not envisioned in this implementation.
Another prior art technique for printing multiple swaths is shown in U.S. patent application Ser. No. 09/233,575, filed Jan. 19, 1999 by common applicant herein Hickman, ENTITLED "DRUM-BASED PRINTER USING MULTIPLE PENS PER COLOR". Ink-jet pens are combined in a printer so that the swaths printed by individual pens are combined into a resultant, wide swath that increases printer throughput. The print medium is carried on a drum and advanced through the printer. Sets of two pens, each set having the same color of ink, are carried near the drum with the two pens arranged such that the swath of one pen is adjacent to the swath of the other pen in a direction that is parallel to the drum axis. Also provided is a carriage assembly for carrying the pens in the just mentioned arrangement for combining the swath widths of the individual pens. The components of the carriage assembly are such that two pens of the same color ink are precisely positioned relative to each other, thereby to meet a very close tolerance requirement for arranging two pens of the same color.
Another prior art technique is to simultaneously print multiple swaths is shown in assignee's co-pending patent application U.S. Ser. No. 09/311,919, filed by D. Pinkernell, ENTITLED "METHOD AND APPARATUS FOR MULTIPLEXED WET-DYE PRINTING", where redundant pen sets are mounted in the y-axis. This also allows simultaneous printing on multiple sheets of paper. Throughput is substantially increased.
There is still a need for ink-jet printer having increased throughput increased color combinations in an apparatus that minimizes product size for a given number of print cartridges in the apparatus.